This invention relates to removing organic coatings, such as paints, lacquers, and the like, from scrap aluminum (including aluminum alloys).
In order to recover coated aluminum scrap, the metal is commonly crushed, shredded, or chopped into smaller pieces. The scrap metal is then decoated, then melted and recovered.
The desirability of decoating prior to melting is well known. Decoating prevents violent gas evolution during melting. It also prevents the evolution of excessive smoke and flame which would otherwise put heavy intermittent demands on fume removal systems. Melting of coated scrap is also associated with large metal losses, which can be in amounts as high as 13% by weight.
A number of processes for decoating scrap metal are known. For example, the treatment of coated scrap metal in kilns, particularly rotary kilns, is well known. Although kilns may be used to remove organic coatings from scrap aluminum, they possess certain disadvantages in terms of effectiveness of heat transfer and rather long residence times. With shredded scrap metal, the major portion of heat transfer to the charge of shredded scrap occurs during the showering of the material through the air prior to its entry into the bed. Relatively little heat transfer actually occurs within the bed. As a result, shredded scrap metal comes up to temperature, usually about 550.degree. C., at different rates and lighter materials are more susceptible to overoxidation because of the longer residence times of the scrap material at elevated temperature within the heating zone of the kiln.
Another known type of scrap decoating treatment involves the use of a vertical, moving packed bed. A vertical, moving packed bed results in better and more efficient heat transfer, but it is also thermally unstable. A further difficulty with vertical, moving bed processes is that high temperatures, e.g. above about 600.degree. C., cause rapid oxidation of aluminum/magnesium alloys and therefore must be avoided. Significant amounts of oxidation may occur and large amounts of heat may be generated, resulting in destructive propagation of a reaction front through the bed.
It is known to subject divided solid material to heat (using blown hot air, for example) with the material arranged in a horizontally moving or travelling bed, e.g. for the sintering of iron ore pellets for subsequent use in blast furnaces; for the treatment of municipal solid waste which includes metallic waste; and for preheating scrap metal prior to melting. Known prior attempts to use such techniques for removing organic coatings from aluminum scrap have not achieved satisfactory decoating of the scrap. However, such attempts used upflow of air through the scrap.